Self-processing photographic products are well known. These products are generally referred to in the photographic art as diffusion transfer, image transfer or instant photographic film units.
In some of these units, an image-receiving layer exhibits a viewable image which can be separated from the image-recording layers after processing. In others, the image-receiving layer can remain permanently attached and integral with the image-recording layer and layers ancillary thereto when a transparent support is employed on the viewing side of the unit. After exposure of the film unit, an alkaline processing composition permeates the various layers to initiate development of the exposed image-recording layers. This processing composition can be introduced from outside the film unit or from a container or rupturable pouch associated with the film unit. In most instances, conventional darkroom processes and after-treatments, such as washing and stabilization are not needed to provide an acceptable image since the film unit is fully self processing.
Usually a self-processing photographic film unit has a thin framing means called a mask. This mask provides a border around an aperture which is adapted to receive the alkaline processing composition. The border also defines an image, or picture area. Typically the mask is located between and around the edges of two components of the film unit (e.g. cover sheet and photosensitive element) to provide a space or cavity between the components for the processing composition. The edge of the mask around the aperture restricts contact of the processing composition to the image area and confines the resulting image to such image area.
The mask is typically bonded to layers or components above and below it with various adhesives. For instance, U.S. Pat. No. 3,256,090 (issued June 14, 1966 to Booth), describes the use of encapsulated, low-strength adhesives to temporarily bond the mask to an image-receiving layer. The described adhesives are either pressure-sensitive, solvent-responsive, chemically-reactive or moisture-releasable adhesives. Other low-strength mask adhesives used to provide temporary bonds are described in U.S. Pat. No. 3,511,659 (issued May 12, 1970 to Dennis, Jr. et al). Those adhesives are mixtures of a vinyl pyrrolidone resin with a copolymer of vinyl acetate (80-90%, by weight) and an acidic monomer (5-20%, by weight). They are degradable by an alkaline processing composition.
Certain acidic polyester mask adhesives which provide permanent bonds in film units are described in U.S. Pat. No. 4,097,282 (issued June 27, 1978 to Noonan et al). Non-acidic polyester mask adhesives are described in U.S. Pat. No. 4,113,493 (issued Sept. 12, 1978 to Sandhu et al).
It has been the aim of workers in the photographic art for a long time to improve border quality in self-processing film units. Border quality denotes relative freedom from visible imperfections in the border around the aperture. Such imperfections include lines, spots, stains and dye deposits. However, these workers have had limited success in improving border quality. What may remove one imperfection may amplify another. The addition of various materials to various layers of the film unit to reduce imperfections is not always possible because such materials may cause sensitometric problems. Some imperfections are caused by manufacturing procedures which cannot be adequately modified to remove the imperfections.
One imperfection in the borders of self-processing film units is known as "border bleed." This imperfection causes the border to appear colored (e.g. cyan) instead of white and to blend into the image area. It is believed that border bleed results from the lateral migration of color-forming dyes or dye-providing materials (e.g. dye releasers) from the image area into the border during or after processing of image-recording layers. Such migration is accelerated when the film unit is subjected to elevated temperatures (e.g. 30.degree.-60.degree. C.) or high humidity after processing for extended periods of time.
Another imperfection observed in some film units after processing has begun is known as "dark edge line." This imperfection is a single or series of parallel dark lines, continuous or segmented, in the border at the edge of the image area. Dark edge line apparently forms in the following way. When alkaline processing composition is introduced into the image area, some of it moves laterally in the image-recording layers into the border "under" the mask. Dyes are caused to be released by the composition as it moves into the border. These dyes diffuse to the image-receiving layer and form objectionable darkened deposits (e.g. dark lines) in the border at the edge of the image area.
Dark edge line is intensified by the presence of air (in the form of "pockets" or "bubbles") which can occur at the border-image area interface. Oxygen within the bubbles causes dye to be released. This dye diffuses to the image-receiving layer and forms dye deposits. The aforementioned air pockets or bubbles can result from nucleation of bubbles at the mask-mask adhesive interface, or from voids in the adhesive. Bubble nucleation and voids can result from nonuniform sealing between mask and image-recording layers which often occurs during conventional manufacturing operations. Air bubbles can also result when the manufactured film units are subjected to stress in handling, shipping and use (e.g. twisting, bending, etc.). Such stressful conditions are very difficult to monitor and control, especially after the film units are in the hands of consumers.
The dark edge line imperfection becomes more pronounced when development is allowed to or necessarily proceeds for relatively longer periods of time (e.g. in the winter time or in colder climates). Yet such long development times may be desirable or necessary in certain situations with certain image-recording materials.
Some reduction in border bleed has been achieved by adding certain acidic terpolymers to gelatin interlayers or titanium dioxide reflective layers of self-processing photographic film units, as described in Research Disclosure, publication 19146, March, 1980 (published by Industrial Opportunities, Ltd., Homewell, Havant Hampshire PO9 1EF United Kingdom). However, this reference fails to teach or suggest how overall border quality can be improved by simultaneously and significantly reducing both border bleed and dark edge line imperfections. Moreover, it has been discovered that placement of the described terpolymers in interlayers or reflective layers may undesirably lengthen access time, i.e. the time required for image formation.
From the foregoing discussion, it is clear that there is a significant problem with imperfections in the borders of image-recording materials (e.g. self-processing film units). An improvement in border quality, particularly with regard to border bleed and dark edge line, would enhance the aesthetic qualities of processed film units (i.e. finished prints) while permitting extended development or storage time of unprocessed film units, if desired.